Crystal Growth Of Phthalocyanines And Study On Fabrications And Performances Of OFET Based On Them

Organic semiconductor materials have good properties such as low cost,structural flexibility, light weight, innocuity, cleanness, portability and lowtemperature processing, which fill in gaps in many areas of inorganicsemiconductor materials. Organic semiconductor materials mainly include small molecular organicmaterials and polymer organic materials. Phthalocyanine is one importantkind of small molecular organic optoelectronic functional material. It is akind of macrocyclic compound with a stable conjugated π system, and it hasgood properties such as heat-resistant and chemical stability that other smallmolecular organic materials don't have. In addition to traditional applicationin organic pigments and dyes industry, many promising applications ofphthalocyanines have emerged recently, such as light emitting diode, thinfilm transistors, photovoltaic batteries, photoconductive materials, chemicaltransducer, laser recording medium, nonlinear optics, and biomedicine for itsnotable semiconductor properties and optoelectronic properties. More than70,000 articles of phthalocyanines have been reported in only half a century.So much research was focused on phthalocyanines, which was moreincomparable than any other compounds.According to the statistics, only 200 kinds of single crystals of nearly10,000 kinds of phthalocyanines compounds have been grown successfullyuntil now, which trailed behind the developments of phthalocyanines industrybadly. The study on the single crystals growth methods is very necessary forthe endless new structures of phthalocyanines compounds.More than 10000 types of phthalocyanines have been discovered until now.But among all these phthalocyanines compounds, only CuPc and itsderivatives are widely used and produced in large quantities in industry. Thusit is very meaningful to study CuPc as a representative. CuPc crystals belongto molecular crystals and they have different forms crystalline. There areeight forms of the CuPc crystals for the time being. Among all the eightforms crystalline, only β-form crystalline has thermo-stability. Other formscrystalline can convert to β-form crystalline at a temperature over 200°C. Inaddition, except for π-form crystalline, crystals of all other forms crystallinecan be made by β-form crystalline directly or indirectly using chemicalmethods. These two points determine a very broad application of β-formCuPc.Now, the methods of growing single crystals of phthalocyanines generallyhave the shortcomings of a long growth period, which accompanies with theinconveniences of low production rate, complex apparatus and difficulties inseparating crystals from medium. A new method of growing β-form singlecrystals of CuPc using flux in a short period with simple devices wasproposed based on a lot of experiments and a series of specific growthtechnology was summarized to grow several phthalocyanines single crystalsquickly and effectively. It solved the difficulties of long growth period, lowproduction rate and the high cost, which also provided a new avenue formaterial purification, preparing single crystals of related organic functionalmaterials and optoelectronic devices based on single crystals. In addition, weoptimized the sublimation-coagulation method and got high-quality singlecrystals.First of all, we designed an automatic set of equipment to observe thegrowth process to grow CuPc single crystals easily. The melted anthraceneused as the solvent in the solution would vaporize and sublime at the top ofthe cuvette. In this process, the liquid level was supersaturated because of theevaporation of the solvent and β-form crystalline CuPc single crystals couldgrow in a short period. We call this method the exaltation-evaporation growthmethod. It is a fast single crystals growth method with simple apparatus, butbiggish crystals can grow easily by it. The growth of single crystals canrealize using only a few solute raw materials (e.g. 10mg) and the productionrate approach 90% that calculated with the weight of the pure CuPc in theraw materials. It also is a cost-effective method because the anthracene as thesolvent can be reused without loss as long as it isn't oxidated. This approachis very meaningful for productions and experiments. It can also be used togrow crystals of other phthalocyanines such as H2Pc, ZnPc, CoPc, MnPc,FePc. The exaltation-evaporation growth method is used as thecomplementarity of the traditional solvent evaporation method and assistedsolvent method, which can be extended to grow crystals of other organicfunction molecules materials.In addition, a lot of CuPc crystals were obtained using thesublimation-coagulation method and the crystals length exceeded 1cm easily.This method is also effective for other phthalocyanines compounds withstable structure and low sublimation temperature. It is suitable for largematerials (above 300mg) purification. Large numbers of larger crystals canbe obtained at a time. Also there are not strict requirements on theenvironment temperature fluctuations in the crystal growth process.After two years of unremitting efforts, we obtained aza-CuPc singlecrystals using the unsealed evaporation method with assisted solvent andanalysed the crystal structure. It is the first single crystal of aza-CuPc in thelast 40 years.During the last twenty years, great developments have been made on thestudy of organic thin film field effect transistors (OFETs) that have beenapplied in active display-driven, large-scale integrated circuits, sensors, andmany other areas. But most devices with high mobility are obtained based oninorganic insulation layer materials such as SiO2, which increases the devicecost and loss the cheap advantage of organic materials. At the same time,chaps appear on the surface for the interior stress at a high temperature annealbecause of the different thermal expansion coefficient between silicon andSiO2. If use the cheap spin-coated polymer thin films, the problem of chapscan be solved and the technologic difficulties can be simplify greatly.Unfortunately, there are hardly any high mobilities obtained from devicesbased on the familiar polymer insulation layer such as PMMA, teflon, nylonetc.A new copolymer of methyl methacrylate and epoxypropyl methacrylate(PMMA-GMA) is used to fabricate a gate insulation layer with a simplespin-coating thin-film processing. After CuPc was purified by thesublimation-coagulation method, CuPc film was deposited by vacuumthermal evaporation to fabricate an active layer. These devices with a simpletop contact structure presented good electrical performances of p-typeenhancement mode field effect transistor. In addition, we optimized devicesparameter of active layer thickness. When the active layer thickness was30nm, the performances of OFET based CuPc become the best. After anneal,the saturated currents and the mobilities increased and the threshold voltagedecreased, which improved the device performance significantly. Highquality pentacene thin film field effect transistors presented good electricalperformances with field-effect mobilities of 0.4cm2 /Vs and low thresholdvoltages of -10V. It was full proof that PMMA-GMA was a competitivecandidate as an excellent gate insulation layer to low the cost and technicaldifficulty. This improved the device performance and provided a widerchoice to fabricate all-organic flexible field effect transistors. In addition, wealso attempted to fabricate field effect transistors using β-form crystallineCuPc crystals as the active layer. PMMA-GMA film not only serves asinsulation layer, but also served as adhesives. The mobilities of devices basedon the single crystals did not exceed the mobilities of devices based onα-form crystalline multi-crystal thin film. We thought it was mainly due tothat the interplanar spacing and "distorted" degree of β-form crystalline CuPccrystals was larger than α-form crystalline CuPc crystals. It decreased thesphere of electronic cloud overlap density, which was not benefit to thetransmission of the carries. At the same time, we proposed our personalopinions on the theory issues and the materials issues in the currentdevelopments of OFET.The optical constants of PMMA-GMA film and CuPc film were measuredby an ellipsometer. In the data fit process, the Cauchy model could be alsoapplied to the absorptive materials through a flexible application of the"Point by Point" fit to simple the fit process. At the same time, the absorptioncoefficients of CuPc film on different crystal type and different thicknesswere measured and the optical band gap could be calculated with the Taucequation from it. We assumed that the changes of the band gap maybe inducethe changes of the carries injection efficiency on the correspondingmetal-organic semiconductor contacts.